Method for the preparation of urea

ABSTRACT

A method for the preparation of urea in a reactor using ammonia and carbon dioxide as starting materials, includes a) bringing the ammonia and carbon dioxide into contact in the reactor under conditions for the formation of carbamate; and b) decomposing the carbamate thus formed to give urea and water. The water formed during step b) is removed from the reaction mixture by a water-selective membrane which is preferably a pervaporation membrane, such as a porous ceramic membrane. Furthermore, a pressure difference is preferably maintained or applied over the selective membrane. The method can in particular be carried out in liquid circulating stream which, in addition to the reactants fed to the circulating stream and the carbamate formed in step a), also contains at least some of the water formed in step b) and also the urea formed in step b), the urea being recovered from the circulating stream and the circulating stream being recycled to the reactor.

This application is a 371 of PCT/NL00/00492 filed Jul. 12, 2000.

BACKGROUND OF THE INVENTION

commercial urea processes consist overall of three process steps, thatis to say synthesis; prilling, granulation or crystallisation; andeffluent treatment and circuits for recycling carbamate to the reactor.

The synthesis step usually comprises two half-)reactions i.e.:

1) Reaction of ammonia with carbon dioxide, which reaction proceedsrapidly and completely to give carbamate in accordance with the reactionequation:

2NH₃+CO₂→NH₂CO₂NH₄

2) The reaction for the dehydration of carbamate to give urea inaccordance with the equation

NH₂CO₂NH₄⇄(NH₂)₂O+H₂O

The latter reaction is an equilibrium reaction and in the usual ureaprocesses achieves approximately 50-60% conversion. The carbamatereaction is highly exothermic and the urea reaction is endothermic.

In modern processes the major proportion of the unconverted carbamate isdecomposed in a steam stripper and, via a condensation step in whichsteam is recovered, is recycled to the reactor. The economy of theseknown processes is highly linked to the yield from the urea synthesisreaction because the latter to a large extent determines how large therecirculation steams are.

In the 1960s and 70s substantial progress was made in the economy ofconventional urea processes by installing a high pressure stripper inthe urea process. With this arrangement, in this stripper a substantialproportion of the unconverted carbamate in the reactor discharge isrecycled, with a limited water content, via a high pressure condensationstep directly to the reactor with the feed (CO₂ or NH₃) to the ureaprocess.

The most important developments for further improvement of the yield inthe urea process the 1980s and 90s can be summarised in the followingmodifications to the reactor section:

produce more plug flow in the reactor

combination of the urea reactor with other process steps

allow the reaction to take place in temperature zones

install a second reactor

remove water via condensation of gas between the reactors.

In Netherlands Patent 1 000 416 the condensation step by means of whichthe high-pressure recycle stream is usually recycled to the reactor iscombined with the reactor. With this arrangement the reactor is orientedhorizontally. The NH₃ is fed into the cooled section of the reactor. Thegas from the stripper is distributed transversely to the liquid streamin the reactor. The intention is that as much urea as possible shouldalready be formed in the cooled section of the reactor. Furthermore, theurea equilibrium in the reactor is better approached by fitting baffles.These prevent back-mixing and give a better approach to plug flow thanthe screen plates in a conventional vertical reactor, as a result ofwhich the synthesis reaction proceeds more rapidly.

EP 0 751 121A2 presents a process in which the urea reaction isdistributed over two temperature zones. One of the zones is atrelatively low temperature, as a result of which the carbamateequilibrium shifts to higher values, and the other temperature zone isat relatively high temperature, as a result of which the ureaequilibrium shifts to higher value. Between the zones water is alsoseparated off by condensation.

EP 0 727 414A1 a process is employed which has an additional reactorunder high pressure and temperature in order to achieve a higherconversion.

In EP 0 624 571 A1 a urea process with high yield is described which hasan additional reactor, water being removed from the feed to the secondreactor by means of condensation. A separate feed control to thisreactor makes it possible to maintain an optimum temperature and NH₃/CO₂ratio.

The process improvements in the urea process have to date been directedtowards achieving an equilibrium for conversion to urea that is asadvantageous as possible and approaching this as closely as possible by,within the limitations which apply for this high pressure andtemperature process, feeding a minimum amount of water to the reactor(s)and optionally carrying out interim water removal between two reactors,choosing the process conditions pressure, temperature and residence timeto be as optimum as possible, choosing a high NH₃ concentration and asfar as possible approaching a plug flow regime in the reactor.

However, during the reaction for the formation of urea a quantity ofwater which is equimolar to the urea produced is formed at the sametime. This substantial quantity of water formed still always ensuresthat the equilibrium for conversion to urea in the reaction is 20-30%below the maximum conversion of 100%.

SUMMARY OF THE INVENTION

The aim of the present invention to improve the known processes and theinvention relates in particular to the removal of water from the ureareactor during the synthesis in order to improve the yield.Specifically, by removing water the equilibrium of the reaction for theformation of urea is shifted towards more extensive conversion to urea.

The removal of this quantity of water formed during the reaction, inorder substantially to increase the conversion to urea in the ureaprocess reactor and to reduce the recycle streams, has to date notproved possible in practice. This is a consequence of the high degree ofdifficulty associated with such a water separation step. An adequateselective water separation step was a technology which did not yet existfor the very high pressure ad temperature conditions in the urea processand the reactor environment, which is highly aggressive from thecorrosive standpoint.

According to the invention a water-selective membrane, in particular apervaporation membrane, is now used to remove the water formed duringthe formation of urea-that is to say during the abovementioned reactionstep 2)—from the reactor, optionally in combination with a pressure dropover said membrane.

In first aspect the invention therefore relates to a method for thepreparation of urea in a reactor using ammonia and carbon dioxide asstarting materials, which method comprises;

a) bringing the ammonia and carbon dioxide into contact in the reactorunder conditions for the formation of carbamate; and

b) dehydrating the carbamate thus formed to give urea and water,characterised in that the water formed during step b) is removed fromthe reaction mixture by the us of a water-selective membrane.

To this end the reaction mixture (or at least the reaction mixture instep b)) is brought into contact in the reactor with one side of theselective membrane, the water formed during step b) being removed fromthe reaction mixture trough said membrane to the other side of themembrane, where it is caught/collected and from where it is removedformed the reactor. During this operation a pressure difference ispreferably maintained or applied over the membrane.

The reaction is preferably carried out in the liquid phase, that is tosay the reactants (in particular ammonia and carbon dioxide in step a)and carbamate in step b)) are mainly, and preferably essentiallyexclusively, in the liquid phase.

As a rule the reaction will be carried out as if in a cyclic process.With is arrangement said cyclic process can comprise a high-pressurecycle and a low-pressure cycle is described in more detail below. (Apossible advantage of the invention could be that the high-pressurecycle can optionally be omitted, as explained in more detail below),

The invention can—with suitable modifications of the equipment used(reactor)—be employed with virtually all known urea processes, includingthe Stamicarbon processes. Reference is made to the known handbooks,such as the “Encyclopedia of Chemical Technology”, Ed. Kirk-Othmer,Wiley Interscience, 3^(rd) Ed (1983), vol. 23, pages 515-561, fordescriptions of these known urea processes.

According to one embodiment (the carbon dioxide stripping process), thecarbon dioxide is fed via a stripper and carbamate condenser to thereactor. The ammonia is fed together with the low-pressure recycle viathe condenser to the reactor.

According to another embodiment (using a so-called pool reactor), theammonia and the carbon dioxide are fed (via the stripper) to saidaqueous liquid stream on entering the reactor after which the mixture isfed through the reactor, whereby urea is formed.

In both embodiments the product stream thus obtained is then removedfrom the reactor ad fed to a stripper, where the carbon dioxide andammonia still present are removed. The latter are then recycled as agaseous stream to the reactor (the “high-pressure cycle”) This canpossibly also still contain residues of carbamate and/or water. (In theconventional processes a carbamate condenser is optionally used in thehigh-pressure cycle; in the case of a pool reactor this carbamatecondenser is integrated in the reactor.) liquid stream which containswater, the—now concentrated—urea and any residues of carbamate stillpresent is also obtained from the stripper. The urea is recovered fromthis stream by means of working-up steps known per se, after which theresidual stream is recycle to the reactor (the “low-pressure cycle”).

The selective membrane used according to the invention is at leastpermeable to water and preferably essentially impermeable to otherconstituents of the reaction mixture. In particular membranes such asare used in so-called “pervaporation” processes, such as, for example,polymer (organic) membranes or ceramic membranes, which can benon-porous, can be used for this purpose. Reference is made to the knownhandbooks, such as Perry's Chemical Engineers Handbook, 7^(th) Ed.McGraw-Hill, 1997, Section 22-67 to 22-69, for a more detaileddescription of such “pervaporation membranes”.

Preferably a porous ceramic membrane is used, that is to say a membranehaving a suitable pore size and thickness, which will be apparent tothose skilled in the art. Pore sizes in range from 0.5 nanometre to 1micrometre and a membrane thickness in the range 1 to 5 mm arepreferred.

Examples of suitable membranes are ceramic membranes made of, forexample, silica or silica/alumina; silicalite (HZSM5); silicalite/ZSM 5zeolite; zeolite A/X (e.g. A4/A5, X13) and palladium-containing ceramicmaterials; and also carbon membranes and membranes made of sinteredstainless steel Such membrane materials can have been further modifiedin a manner known per se, for example by means of ion exchange (forexample sodium for copper, etc.), by means of surface treatment and thelike. Examples of these and other suitable membranes will be apparent tothose skilled in the art, with regard to which reference is again madeto the description of pervaporation processes in the known handbooks.

The membranes are preferably suitable for use at the operatingtemperature for the process ad against any pressure drop applied overthe membrane and also against the constituents of the reaction mixture.

It is found that adequate selective removal of water during the ureasynthesis in the reactor be achieved by the use of these membranes,integrated in the urea reactor in an adequate manner. The majordifficulties which existed for performing selective water removal insitu during the urea reaction are solved by means of this method andprocess design. By employing this method and process design it ispossible to increase the conversion to urea per reactor pass by 10-20%.

The membrane can be of any suitable shape and size and can, for example,be a flat membrane a spirally wound membrane, a “plate and frame” moduleor a hollow fibre membrane. In this context the membrane is preferablyso constructed that it defines, at at least one side thereof, adischarge chamber or discharge channel that is essentially closed off(from reaction chamber), that is to say on its own, in combination withone or more other membranes present and/or in combination with otherelements of the reactor, such as the wall(s of the reactor or baffles,plates or trays present in the reactor. During operation of the reactorone side of the membrane is in contact with the reaction mixture, whilstthat side of the membrane which forms the discharge channel or thedischarge chamber is effectively connected to a discharge line, by meansof which the water removed from the reaction mixture can be dischargedfrom the reactor.

The Factor can be provided with a single membrane module (i.e. membraneand discharge ine/discharge chamber) or with multiple modules, whichoptionally can be connected or joined to one another. The total surfacearea of the membranes will depend on the size of the reactor and theconversion to be achieved. According to a preferred embodiments thereactor is divided into two or more segments by one or more baffles,plates or trays provided in the rector, each segment optionally beingprovided with its own member module, the reactants essentially beingsupplied at one end of the reactor, after which the reaction mixtureruns through the various segments in the reactor and is then removedfrom the reactor at the other end. This prevents back-mixing andpromotes the formation of a plug flow. With this arrangement the reactorcan be positioned either essentially horizontally or essentiallyvertically; furthermore a temperature gradient can be maintained orapplied over the segments of the reactor.

The design of the reactor, and in particular the positioning of themembranes, baffles, trays and walls, is furthermore preferably such thatthe rising gas in the urea reactor gives rise to turbulent flow in thereactor during operation, in particular at the membrane surface on thereactor side.

Since the reaction is preferably carried out at elevatedtemperature—usually in the range from 150-250° C.—and elevatedpressure—usually in the range from 100 to 200 bar —the reactor ispreferably constructed as an essentially sealed pressure vessel providedwith the one or more membrane units and suitable feed and dischargelines. The reactor can furthermore contain all elements of urea reactorsthat are known per se, such as heating element, mixing and stirringunits, cooling elements, measurement and control equipment and the like.

Suitable reactor designs will be apparent to those skilled in the art.In practice it will be possible to make use of a reactor known per sefor the synthesis of urea which is provided with one or more membranemodules in accordance with the invention.

During operation water is withdrawn from the reaction zone through themembranes until the desired conversion has been reached, and preferablyessentially continuously. To this end pressure difference is preferablyapplied over the membrane—for example with a pressure drop in the rangeof 50-200 bar—with the high(er) pressure on the reaction mixture side.To this end the discharge side of the membrane can be effectivelyconnected to a vacuum pump, optionally in combination with a cooler.

The quantity of water which is removed from the reactor during thereaction is preferably such that the carbamate decomposition step in theoutlet from the urea reactor offers no further advantages and the CO₂,feed gas can be fed directly to the reactor, omitting this carbamatedecomposition step. This means that the abovementioned reaction steps a)and b) can essentially be carried out in a single reactor.

The temperature of the reactor, and in particular the beat required forthe pervaporation, is preferably essentially obtained/maintained by thecarbamate reaction of feed components to, the reactor and/or bycondensation of the stripping gas supplied.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained on the basis of the followingdescription and the non-limiting FIGS. 1 and 2, which show preferredembodiment of the urea reactor according to the invention—with selectivemembranes for the removal of water. More particularly:

FIG. 1 shows a horizontal urea reactor with heat removal and installedpervaporation membranes; and

FIG. 2 shows a conventional vertical urea reactor with heat removal andinstalled pervaporation membranes.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a high-pressure section of a urea plant with a horizontalreactor. In FIG. 1 (A) indicates a horizontal reactor with interpervaporation membranes, condensation zone and heat exchanger, (B)indicates a stripping zone, (C) indicates a steam reservoir, (H)indicates a condenser and (D), (E), (F), (G) and (I) indicate (vacuum)pumps and compressors.

By means of pump (E) liquid ammonia is fed via line (1) and via a line(4) provided with openings to the condensation zone of the reactor. Acarbamate solution that has been obtained elsewhere in the process,specifically by washing off-gases with an aqueous solution which hasbeen obtained on evaporation of the urea solution, is drawn in via line(2). The ammonium carbamate solution is fed via line (3) to the reactor(A). A mixture containing ammonia and carbon dioxide is fed into theliquid via line (5) provided with openings. This gas mixture, suppliedvia line (15), has been obtained by subjecting the urea synthesissolution formed in the reactor to a stripping treatment in the strippingzone (B) wit the supply of heat and in counter-current to a strippinggas via line (13), for example carbon dioxide. In the embodiment shown,the pressures in rector (A) and the stripping zone (B) are virtuallyidentical, for example 140 bar. The pressures and temperatures in thesaid zones can, however, also differ from one another. The reactor isfurthermore provided with baffles, which divide the reactor intocompartments.

The removal of water from the reactor takes place via the (ceramic)pervaporation membrane modules (19) which are positioned in thecompartments between the baffles (in Figure five such modules are shown;this number can be higher or lower, depending on the size of theerector), upstream of the last but one compartment, in which tapping iscontrolled by means of the reactor level. The positioning of thesemodules is above line (5) such that the gas comes thoroughly intocontact with the liquid which is flowing ever the membranes and ensuresgood mixing. The water removed via the (ceramic) membranes is fed via acondenser (H) using a (vacuum) pump (I). The vapour, mainly consistingof water, is condensed in this condenser (H). The condensed steam fromthe condenser (21) and the vapour stream from the vacuum pump (22) are,for example, fed to a stripper in the effluent treatment plant. Theeffluent treatment plant for the urea process has not been drawn here.

The heat liberated in the reactor is removed with the aid of water,supplied via line (6), which is fed by means of pump (G) via (7) throughthe heat exchanger (8) mounted in the reactor (A) and which during thisoperation is converted into low-pressure steam. The steam formed is fedvia line (9) into steam reservoir (C) and discharged from the latter vialine (10) to installation, which is not shown, which uses low-pressuresteam, for example the recirculation and/or evaporation section. Heat issupplied to the water-removing and heat-removing pervaporation membranes(19) by means of heat which is supplied by carbamate formation in thereactor feed and a targeted supply of condensing stripper gas via line(15) and correct dimensioning of distributor (5) to provide the correctdistribution.

The inert gases, which additionally also contain ammonia and CO₂, aredischarged from the reactor (A) via lie (14). NH₃ and CO₂ are removedfrom these gases in a known manner. The urea synthesis solution is fedfrom the reactor (A) via line (11) to the stripping zone (B). Thestripped urea synthesis solution is discharged via line (12) and furtherprocessed in a known manner to give an aqueous urea solution andconcentrated, after which the concentrated solution is optionallyconverted to solid urea.

FIG. 2 shows a conventional vertical urea reactor with a heat dischargeand installed pervaporation membranes.

FIG. 2 shows the high-pressure section of a conventional urea processwith a vertical reactor (A). The condensation step for the strippedcarbamate stream (4) from the stripper (B) is shown in (C). Thepartially condensed stream from condenser (C) is fed into the bottom ofthe vertical reactor (A).

The overflow from reactor (A), which contains urea and unconvertedcarbamate, is subjected via (6), to the stripping treatment, resultingin a stripped stream (4) and a urea solution which is optionallyconverted to solid urea in a known manner.

The reactors described above can, for example be designed for theproduction of 1,500 m urea/day.

FIG. 1 shows the reactor (A) and stripper (B) and a few further piecesof equipment thereof, Via stream 21 20 tonnes/h water is discharged fromreactor by means of the (vacuum) pump (I) and condenser (H) via theceramic pervaporation elements 19 which have been installed between thevarious segments in the reactor.

Compared with known processes, inter alia the following advantages areachieved with the invention:

shift in the equilibrium towards the formation of urea, as a result ofwhich a higher yield is obtained;

lower energy consumption, inter alia as a result of;

60% reduction in import of expensive medium-pressure steam

no further export production of export low-pressure steam which isdifficult to sell and yields virtually nothing

30% less evaporation is required in the urea evaporation step

the load to the medium-pressure recycle is reduced by, 15%;

reduction in the dimensions of the equipment to be used. For instance,the reactor volume and the heat-exchange surface area can be reduced by30%, the surface area of he stripper (B) can be reduced by 35% and thesurface area of the carbamate condenser can be reduced by 50%.

What is claimed is:
 1. Method for the preparation of urea from areaction mixture in a reactor using ammonia and carbon dioxide asstarting materials, which method comprises a) bringing the ammonia andcarbon dioxide into contact in the reactor under conditions for theformation of ammonium carbamate; and b) dehydrating the ammoniumcarbamate thus formed to give urea and water, wherein the reactionmixture is brought into contact in the reactor with one side (reactionmixture side) of a water-selective membrane, the water formed duringstep b) being removed from the reaction mixture through said membrane tothe other (discharge) side of the membrane, characterised in that themembrane is a pervaporation membrane and that a pressure difference isapplied over the membrane with the higher pressure on the reactionmixture side, the water being removed as a vapour.
 2. Method accordingto claim 1, wherein the discharge side of the membrane is connected tovacuum pump.
 3. Method according to claim 1, wherein the water-selectivemembrane is a porous ceramic membrane.
 4. Method according to claim 1,wherein the formation of urea is carried out in a cyclic process. 5.Method according to claim 1, wherein a reactor is used which is providedwith one or more baffles, plates or trays which divide the reactor intotwo or more segments, each segment preferably being provided with awater-selective pervaporation membrane.
 6. Method according to claim 1,wherein the design of the reactor, and in particular the positioning ofthe membranes, baffles, trays and walls in the reactor, is such thatduring operation the rising gas in the reactor gives rise to turbulentflow in the reactor, in particular at the surface of the one or moreceramic membranes.
 7. Method according to claim 1, wherein the quantityof water that is removed from the reactor during operation is such thatthe abovementioned reaction steps a) and b) essentially can be carriedout in a single reactor and the CO₂ feed gas can be fed directly to thereactor.
 8. Method according to claim 1, wherein the temperature of thereactor, and in particular the heat required for the pervaporation, isessentially obtained/maintained by the carbamate reaction of feedcomponents to the reactor and/or by condensation of the stripper gassupplied.
 9. Method according to claim 2 wherein the discharge side ofthe membrane is also connected to a condenser to condense the vapors.